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<a name="Function-Prototypes"></a>
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<hr>
<a name="Prototypes-and-Old_002dStyle-Function-Definitions"></a>
<h3 class="section">6.40 Prototypes and Old-Style Function Definitions</h3>
<a name="index-function-prototype-declarations"></a>
<a name="index-old_002dstyle-function-definitions"></a>
<a name="index-promotion-of-formal-parameters"></a>

<p>GNU C extends ISO C to allow a function prototype to override a later
old-style non-prototype definition.  Consider the following example:
</p>
<div class="smallexample">
<pre class="smallexample">/* <span class="roman">Use prototypes unless the compiler is old-fashioned.</span>  */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif

/* <span class="roman">Prototype function declaration.</span>  */
int isroot P((uid_t));

/* <span class="roman">Old-style function definition.</span>  */
int
isroot (x)   /* <span class="roman">??? lossage here ???</span> */
     uid_t x;
{
  return x == 0;
}
</pre></div>

<p>Suppose the type <code>uid_t</code> happens to be <code>short</code>.  ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted.  Therefore in this example the
function definition&rsquo;s argument is really an <code>int</code>, which does not
match the prototype argument type of <code>short</code>.
</p>
<p>This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the <code>uid_t</code> type is <code>short</code>, <code>int</code>, or
<code>long</code>.  Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition.  More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion.  Thus in GNU C the above example is
equivalent to the following:
</p>
<div class="smallexample">
<pre class="smallexample">int isroot (uid_t);

int
isroot (uid_t x)
{
  return x == 0;
}
</pre></div>

<p>GNU C++ does not support old-style function definitions, so this
extension is irrelevant.
</p>



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